The present invention relates to a semiconductor device and manufacturing method thereof, and more particularly to a semiconductor device including a wiring layer and method for forming a wiring layer. The present invention is an improvement over the invention which is the subject matter of the present inventor's co-pending U.S. patent application Ser. No. 07/828,458 filed on Jan. 31, 1992, the disclosure of which is hereby incorporated into this application by reference.
The metallization process is regarded as being the most important matter of semiconductor device manufacturing technology, since it increasingly determines the yield, performance (e.g., speed of operation), and reliability of the devices, as the technology advances toward ultra large-scale integration (ULSI). Metal step coverage was not a serious problem in less dense prior art semiconductor devices, because of the inherent features of devices having larger geometries, e.g., contact holes having low aspect ratios (the ratio of depth to width), and shallow steps. However, with increased integration density in semiconductor devices, contact holes have become significantly smaller while impurity-doped regions formed in the surface of the semiconductor substrate have become much thinner. Due to the resulting higher aspect ratio of the contact holes and larger the steps, with these current, grater-density semiconductor devices, it has become necessary, to improve the conventional aluminum (Al) metallization process, in order to achieve the standard design objectives of high-speed performance, high yield, and good reliability of the semiconductor device. More particularly, the utilization of the conventional Al metallization process in the fabrication of the higher-density integrated semiconductor devices has resulted in such problems as degraded reliability and failure of the Al interconnections due to the high aspect ratio of the contact holes and poor step coverage of the sputtered Al, increased contact resistance caused by silicon (Si) precipitation, and degradation of the shallow junction characteristics due to Al spiking.
In an effort to overcome these problems of the conventional Al metallization process, various new processes have been proposed. For example, for preventing degraded semiconductor reliability caused by the above-mentioned failure of Al interconnections ,the following processes have been proposed.
Melting methods have been disclosed in such patent publications such as Japanese Laid-Open Publication No. 62-132348 (by Yukiyosu Sugano et al.), Japanese Laid-Open Publication No. 63-99546 (by Shinpei Iijima), Japanese Laid-Open Publication No. 62-109341 (by Masahiro Shimizu et al.), Japanese Laid-Open Publication No. 62-211915 (by Hidekazu Okabayashi et al.), Japanese Laid-Open Publication No. 1-246831 (by Seiichi Iwamatsu), Japanese Laid-Open Publication No. 59-171374 (by Masaki Satou) and European Patent Application No. 87306084.3 (by Ryoichi Mukai et al.).
According to the above method, the contact hole is filled by means of melting and reflowing Al or an Al alloy. To summarize, in the reflowing step, the metal layer of Al or Al alloy is heated beyond its melting temperature, and the thus melted metal is flowed into the contact hole to fill the same. This reflowing step entails the following drawbacks and disadvantages. First of all, the semiconductor wafer must be disposed horizontally so as to allow proper filling of the contact hole with the flowing melted material. Secondly, the liquid metal layer flowed into the contact hole will seek a lower surface tension, and thus, may, upon solidifying, shrink or warp, and thereby expose the underlying semiconductor material. Further, the heat treatment temperature cannot be precisely controlled and therefore, given results are difficult to reproduce. Moreover, although these methods may fill a contact hole with the melted metal of the metal layer, the remaining areas of the metal layer (outside of the contact hole area) may become rough, thereby impairing subsequent photolithography processes. Therefore, a second metallization process may be required to smooth or planarize these rough areas of the metal layer.
As an alternative to melting Al or Al alloy for filling contact holes, and in order to improve the metal step coverage, a multiple step metallization process is disclosed in U.S. Pat. No. 4,970,176 (Clarence J. Tracy et al.). According to this above patent, a predetermined first thickness of a metal layer is deposited on a semiconductor wafer at a cold temperature. Then, the temperature is increased to approximately 400.degree. C. to 500.degree. C., which allows the metal layer to reflow while depositing the remaining and relatively thin second thickness of the metal layer. The reflow of the metal layer takes place through grain growth, recrystallization and bulk diffusion.
According to the Tracy et al. method, the step coverage of a contact hole (via hole) having a high aspect ratio can be improved. However, the aluminum or aluminum alloy cannot completely fill a contact hole having an aspect ratio greater than 1 and a diameter less than 1 .mu.m.
In the meantime, Ono et al. have disclosed that when the semiconductor substrate temperature is above 500.degree. C., the liquidity of Al--Si suddenly increases (in Proc., 1990 VMIC Conference, June 11 and 12, pp. 76-82). According to this paper, the stress of an Al-1% Si film changes abruptly near 500.degree. C., and the stress relaxation of such a film occurs rapidly at that temperature. Additionally, the temperature of the semiconductor substrate must be maintained between 500.degree. C. and 500.degree. C. in order to fill the contact holes satisfactorily. This mechanism is different from the reflow of the metal layer in the Tracy et al. patent ('176).
Additionally, C. S. Park et al. (which includes some of the present inventors) have disclosed a method for forming a metal wiring layer through a contact hole having a high aspect ratio which comprises the steps of depositing an aluminum alloy to a thickness of 3000 .ANG. at a temperature below 100.degree. C. and post-heating the deposited aluminum alloy at a temperature of 550.degree. C. for 180 seconds to thereby completely fill up the contact hole with aluminum alloy, in Proc., 1991 VMIC Conference, June 11 and 12, pp. 326-328. This method is now pending in the USPTO as U.S. patent application Ser. No. 07/585,218 entitled "A :Method for Forming a Metal Layer in a Semiconductor Device".
Since the metal layer is heat-treated at a temperature lower than aluminum's melting point, the metal layer does not melt. For example, instead of melting, the Al atoms deposited by sputtering at a temperature below 150.degree. C. migrate upon heat-treatment at 550.degree. C. This migration increases when the surface area is uneven or grainy due to an increase in energy among the surface atoms which are not in full contact with surrounding atoms. Thus, the initially sputtered, grainy layer exhibits increased atomic migration upon heat-treatment.
According to the above method, the contact hole can be easily and fully filled with metal, by using the same sputtering equipment used for the conventional deposition method and then annealing the deposited metal. Therefore, even a contact hole with a high aspect ratio can be completely filled.
However, when a void is formed in the contact hole or when the step coverage of the metal layer is inadequate, the contact hole cannot be filled up while maintaining such a semiconductor wafer with metal layer at a certain temperature and vacuum level. Further, although a secondary metal layer is subsequently formed on the semiconductor wafer having a previously deposited primary metal layer, good step coverage of the contact hole cannot be assured, and the reliability of the manufactured semiconductor device is degraded due to this inadequate step coverage.
A contact structure consisting of pure Al deposited directly onto Si was adopted in the earliest stages of silicon technology. However, the Al--to--Si contact exhibits some poor contact characteristics such as junction spiking during sintering. The sintering step is performed after the contact metal film has been deposited and patterned. For Al-to-Si contacts, such sintering causes the Al to react with the native oxide layer that forms on the silicon surface. As the Al reacts with the thin SiO.sub.2 layer, Al.sub.2 O.sub.3 is formed, and in a good ohmic contact, the native oxide is eventually completely consumed. Thereafter, Al diffuses through the resultant Al.sub.2 O.sub.3 layer to reach the Si surface, forming an intimate metal-Si contact. Here, Al must diffuse through the Al.sub.2 O.sub.3 layer to reach the remaining SiO.sub.2. As the Al.sub.2 O.sub.3 layer increases in thickness, the Al penetration requires more time. Thus, if the native-oxide layer is too thick, the Al.sub.2 O.sub.3 layer will consequentially too thick for Al to diffuse through it. In this case, not all of the SiO.sub.2 will be consumed, and a poor ohmic contact will result. The penetration rate of Al through Al.sub.2 O.sub.3 is a function of temperature. For acceptable sinter temperature and sinter times, the thickness of the Al.sub.2 O.sub.3 should be in the range of 5-10 .ANG.. Since the maximum Al.sub.2 O.sub.3 thickness is of the order of the consumed native oxide's thickness, an approximate upper limit to the allowable thickness of the native-oxide layer is fixed. The longer the silicon surface is exposed to an oxygen-containing ambient atmosphere, the thicker the native oxide will be. Therefore, in most contact processes, surface-cleaning procedures are performed just prior to loading the waters into the deposition chamber for metal deposition.
Aluminum absorbs 0.5 to 1% silicon at a contact-alloying temperature between 450.degree. C. and 500.degree. C. If a pure Al film were heated to 450.degree. C. and a source of silicon were provided, then the Al would absorb silicon in solution until a Si concentration of 0.5 percent by weight is reached. The semiconductor substrate serves as such a source of silicon, while silicon from the substrate enters the Al by diffusion, at elevated temperatures. If a large volume of Al is available, a significant quantity of the Si from below the Al--Si interface can diffuse into the Al film. Simultaneously, the Al from the film moves rapidly to fill the voids created by the departing Si. If the penetration of the Al is deeper than the pn-junction depth below the contact, the junction will exhibit large leakage currents or even become electrically shorted. This phenomenon is referred to as junction spiking.
For alleviating the problem of junction spiking at the contacts, Si is added to the Al film as it is deposited. Aluminum-silicon alloys (1.0 wt % Si) have been widely adopted for manufacturing the contacts and interconnects of integrated circuits. The use of aluminum-silicon alloys instead of pure Al may alleviate the problem of junction spiking, but, unfortunately, causes another problem. More particularly, during the cooling cycle of the annealing process, the solubility of silicon in the Al decreases with the decreasing temperature. The aluminum thus becomes supersaturated with Si, which causes the nucleation and outgrowth of Si precipitates from the Al--Si solution. Nucleation should always occur most rapidly on the grain boundaries and interface boundaries in the decreasing order of the driving force for nucleation. Such precipitation occurs both at the Al--SiO.sub.2 interface and Al--Si interface in the contacts. If these precipitates form n.sup.+ Si at the contact interface, an undesirable increase in contact resistance results. Si precipitates on grain boundaries can take part in boundary-assisted nucleation and those within the Al interconnect lines can increase the susceptibility of the lines to electromigration failure. A large flux divergence in current is produced at locations where Si precipitates larger than approximately 0.4 .mu.m are formed. This can lead to early failure of the semiconductor device due to an electromigration-induced open circuit. When forming a metal wiring layer in a semiconductor device according to the above method (that of C. S. Park), this problem becomes serious since the metal wiring layer undergoes a heating and cooling cycle during its formation.
FIG. 1. illustrates Si precipitates (8a, 8b) formed on the surface of the semiconductor substrate 2 after metallization. Here, reference numeral 7 represents the metal wiring layer. Obviously, these Si precipitates should be removed. These Si precipitates have hitherto been removed by ashing, overetching or wet etching, or by the use of an enchant including a radical which can remove the precipitates from the substrate.
In particular, when depositing the metal layer at a high temperature, the Si precipitates cannot be easily removed. When the Si precipitates are removed by overetching, the images thereof are transmitted to an underlying layer, and these images remain after the overetching. Thus, the quality and appearance of the surface of the semiconductor substrate remains poor.
It is also presently known that, for improving the reliability of the semiconductor by preventing degradation of the shallow junction characteristics due to Al spiking, a barrier layer can be formed in the contact hole formed on the semiconductor water. For example, the formation of a titanium nitride film by a reactive sputtering method is disclosed in J. Vac. Sci. Technol., A4(4), 1986, pp. 1850-1854. In U.S. Pat. No. 4,897,709 (by Natsuki Yokoyama et al.), there is described a semiconductor device which includes a titanium nitride film (barrier layer) which is formed in a contact hole for preventing a reaction between the metal wiring layer and the semiconductor substrate. The titanium nitride film can be formed by a low pressure CVD method implemented with a cold-type CVD apparatus. The resultant film has excellent characteristics with good step coverage for a considerably fine hole having a large aspect ratio. After forming the titanium nitride film, a wiring layer is formed by a sputtering method using an Al alloy.
Additionally, Yoda Dakashi et al. have suggested a method for manufacturing a semiconductor device which comprises the steps of forming double barrier layers for preventing a reaction between the wiring layer and the semiconductor substrate or an insulation layer, on the inner surface of the contact holes, and then filling the contact holes with a deposited metal such as an Al alloy while heating the semiconductor substrate to a desired temperature (Korean Laid-open Patent Publication No. 90-15277 corresponding to Japanese Patent Application No. 01-061557 filed on Mar. 14, 1989.)
Additionally, in a Japanese Patent Laid-open Publication No. 61-183942, there is described a method for forming a barrier layer which comprises the steps of forming a metal layer by depositing a metal such as Mo, W, Ti or Ta, forming a titanium nitride layer on the metal layer, and heat-treating the metal layer and the titanium nitride layer to thereby form a metal silicide layer by a reaction between the metal layer and semiconductor substrate at the intersurface thereof. Thus, the barrier characteristic is improved. However, merely forming a barrier layer is insufficient for overcoming the shortcomings and disadvantages of the above C. S. Park's metallization process.
For overcoming the above problems, S. I. Lee (also one of the present inventors) et al. has an invention now pending in the USPTO entitled "Method for Manufacturing a Semiconductor Device," and filed as U.S. patent application Ser. No. 07/828,458. This invention relates to a method for forming a metal wiring layer through a contact hole in a semiconductor device, which comprises the step of forming a first metal layer on a semiconductor wafer coated with an insulating layer having a contact hole formed thereon, using a metal selected from a group consisting of pure Al and aluminum alloys having no Si component, heat-treating the metal layer to completely fill up the contact hole with a metal of the first metal layer and then forming a second metal layer having a Si component on the first metal layer.
FIGS. 2 to 5 show a method for forming a metal wiring layer according to the above invention.
FIG. 2 illustrates a step of forming a first metal layer. More particularly, an opening 23 having a 0.8 .mu.m diameter and having a stepped portion thereon is formed on a semiconductor substrate 21 provided with an insulating interlayer 22. Thereafter substrate 21 is cleaned.
Next, a diffusion barrier layer 24 consisting of a high-melting temperature metal compound such as TiN is deposited over the entire surface of insulating interlayer 22 and exposed portions of semiconductor substrate 21. The thickness of barrier layer 24 is preferably between 200-1500 .ANG.. Semiconductor substrate 21 is then put into a sputtering reaction chamber, wherein a first metal layer 25 is formed by depositing a metal, e.g., aluminum or an aluminum alloy with no Si component, to a thickness of two-thirds of the desired thickness of the total (composite) metal layer (4000 .ANG. when the desired thickness of the total metal layer is 6000 .ANG.), at a temperature below 150.degree. C. and under a predetermined vacuum level. First metal layer 25 thus formed has small aluminum grains and a high surface free energy.
FIG. 3 illustrates a step of filling openings 23. More particularly, the semiconductor wafer is moved into another sputtering reaction chamber without breaking the vacuum, wherein first metal layer 25 is heat-treated, preferably at a temperature of 550.degree. C. for 3 minutes, thereby causing the atoms of aluminum to migrate into opening 23. The migration of the aluminum atoms causes the surface free energy thereof to be reduced, thereby decreasing its surface area and facilitating the completely filling of the openings with the aluminum, as shown in FIG. 3.
FIG. 4 illustrates a step of forming a second metal layer 26 on first metal layer 25. More particularly, second metal layer 26 is formed by depositing the remainder of the required thickness of the total metal layer at a temperature below 350.degree. C., thereby completing the formation of the total metal layer. Second metal layer 26 is formed by using an aluminum alloy having a Si component, such as Al--Si or Al--Cu--Si.
FIG. 5 illustrates a metal wiring pattern 27 obtained by removing predetermined portions of second metal layer 26, first metal layer 25 and barrier layer 24, by a conventional lithography process, such as is well-known in the field of semiconductor processing.
Further, according to the invention described in the above U.S. patent application Ser. No. 07/828,458, the above second metal layer 26 may be heat-treated in the same manner as first metal layer 25, to thereby planarize the surface of the metal layer in order to improve a subsequent photolithography process before forming a metal wiring pattern 27.
According to the above invention, a metal with no Si component and a metal with a Si component are successively deposited to form a composite metal layer. A metal layer with no Si component absorbs Si atoms from the metal with the Si component, when the temperature of the semiconductor substrate is lowered. Therefore, Si precipitates are not formed on the surface of the semiconductor substrate after forming the wiring pattern.
However, when forming a composite metal layer, pure aluminum or an aluminum alloy having no Si component is deposited to form a first metal layer and then an aluminum alloy having a Si component is deposited to form a second metal layer. Therefore, if there exists a poor diffusion barrier layer formed on the inner surface of a contact hole, a fine junction spiking 15 occurs as shown in FIG. 6. Here, reference numeral 13 represents an impurity-doped region. Thus, the junction is deteriorated, which, over time, will increase leakage current.
Based upon the above, it can be appreciated that there presently exists a need for a semiconductor device including a wiring layer which does not produce a Si precipitate nor a fine Al spiking which causes leakage current and a manufacturing method thereof, which would overcome the above-described shortcomings and disadvantages of the presently available processes. The present invention has been accomplished for fulfilling this need.